The invention is directed to a process for increasing the corrosion resistance of nitrided structural parts made of iron materials by an oxidation treatment carried out in connection with the nitriding.
By nitriding structural parts made of iron materials there is also increased their resistance to corrosion with the exception of rust and acid resistant steels. This state of affairs has been known for a very long time. The effect occurring is independent of the nitriding process (salt bath, short time gas, powder, plasma nitrided) used. The only exception is the so-called classical gas nitriding in ammonia in which generally the compound zone formed is worked off.
In the industrial use of the nitriding for the purpose of improving the welding properties and increasing the fatigue strength, as it is practised at present, the improvement of the corrosion properties are regarded as welcome side effects. However, there is no known case in which the nitriding is used exclusively for the purpose of increasing the corrosion resistance. Therefore, there are known efficient methods, e.g. chromeplating and the like.
Under the designation "special blues" there is known in the art a combination of nitriding and steam tempering. This process serves exclusively for the improvement of the welding properties of chill casting and consists of a combination of the so-called classical gas nitriding with a steam tempering at relatively high temperature. Furthermore it is also known that this treatment leads to an improved oxidation resistance. The known process, however, is only usable to a very limited extent, namely only for the mentioned group of work materials.
Therefore, it was the problem of the invention to find a process for increasing the corrosion resistance of nitrided structural parts made of iron materials by an oxidation treatment in connection with the nitriding process which is usable for all iron material.